Add narrow.narrow_to_next_pm_string().
Add narrow.narrow_to_next_pm_string().

This helper function will allow us to cycle through PM narrows
that are unread, once we map it to a hotkey and/or other UI.

(We intend to make something like the "n" key for topics, but
that works on PM narrows instead.)

rht/zulip,rishig/zulip,jackrzhang/zulip,dhcrzf/zulip,jackrzhang/zulip,timabbott/zulip,rishig/zulip,shubhamdhama/zulip,zulip/zulip,rishig/zulip,hackerkid/zulip,punchagan/zulip,dhcrzf/zulip,andersk/zulip,rishig/zulip,andersk/zulip,synicalsyntax/zulip,timabbott/zulip,kou/zulip,zulip/zulip,hackerkid/zulip,brainwane/zulip,showell/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,rishig/zulip,tommyip/zulip,kou/zulip,rht/zulip,andersk/zulip,brainwane/zulip,showell/zulip,brainwane/zulip,showell/zulip,timabbott/zulip,tommyip/zulip,timabbott/zulip,punchagan/zulip,dhcrzf/zulip,showell/zulip,showell/zulip,jackrzhang/zulip,jackrzhang/zulip,zulip/zulip,dhcrzf/zulip,andersk/zulip,kou/zulip,brainwane/zulip,timabbott/zulip,showell/zulip,brainwane/zulip,shubhamdhama/zulip,jackrzhang/zulip,eeshangarg/zulip,rishig/zulip,tommyip/zulip,jackrzhang/zulip,punchagan/zulip,eeshangarg/zulip,rht/zulip,rishig/zulip,synicalsyntax/zulip,synicalsyntax/zulip,shubhamdhama/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,zulip/zulip,brainwane/zulip,tommyip/zulip,hackerkid/zulip,tommyip/zulip,hackerkid/zulip,dhcrzf/zulip,dhcrzf/zulip,hackerkid/zulip,rht/zulip,timabbott/zulip,shubhamdhama/zulip,showell/zulip,punchagan/zulip,punchagan/zulip,synicalsyntax/zulip,synicalsyntax/zulip,synicalsyntax/zulip,jackrzhang/zulip,rht/zulip,eeshangarg/zulip,shubhamdhama/zulip,synicalsyntax/zulip,tommyip/zulip,andersk/zulip,dhcrzf/zulip,andersk/zulip,shubhamdhama/zulip,punchagan/zulip,tommyip/zulip,eeshangarg/zulip,brainwane/zulip,eeshangarg/zulip,kou/zulip,punchagan/zulip,hackerkid/zulip,timabbott/zulip,kou/zulip,kou/zulip,andersk/zulip,hackerkid/zulip,shubhamdhama/zulip,zulip/zulip